This invention relates to an exposure apparatus which uses vacuum ultraviolet rays having a wavelength not longer than 160 nm, as an exposure beam. Also, the invention concerns a semiconductor device manufacturing method or a semiconductor manufacturing factory using such an exposure apparatus, or a maintenance method therefor.
In projection exposure apparatus for the manufacture of semiconductor integrated circuits, light of various wavelength regions is projected to a substrate as an exposure beam. As regards the exposure beam, for example, g-line (436 nm), i-line (365 nm), a KrF excimer laser (248 nm) or an ArF excimer laser (193 nm) is used.
The exposure beam emitted from a light source is directed through an illumination optical system for illuminating a reticle (or mask) and a projection optical system (projection lens) for imaging a fine pattern formed on the reticle upon a substrate such as a semiconductor wafer substrate, for example, by which the fine pattern is lithographically transferred to a photosensitive substrate. In such conventional exposure apparatuses, further improvements in throughput and resolution are required to meet further miniaturization of the pattern linewidth. In this connection, an exposure beam having a higher power is desired and, on the other hand, the shortening of the wavelength of the exposure beam is required.
The pattern linewidth of next generation semiconductor integrated circuits will be about 100 to 70 nm. As regards the exposure beam wavelength region, an F2 excimer laser having a wavelength of 157 nm, shorter than that of an ArF excimer laser, is expected.
However, it is known that, when a short wavelength exposure beam of an i-line or one shorter than it is used, the short wavelength exposure beam causes a photochemical reaction of impurities and oxygen in the air. The product of such a reaction is deposited on an optical element (lens or mirror) of the optical system to cause a decrease in characteristics such as optical efficiency, for example. This leads to a decrease in throughput of the exposure apparatus.
As regards the product, representative examples are ammonium sulfate (NH4)2O4 to be produced, when sulfurous acid SO2 absorbs light energy and is excited thereby, through reaction (oxidation) of the same with oxygen in the air, and SiO2 to be produced, when a Si compound absorbs light energy and is excited thereby, through reaction of the same with oxygen in the air.
Conventionally, in order to avoid such a product, an optical system is purged by an inactive gas. For example, Japanese Laid-Open Patent Application, Laid-Open No. 216000/1994 shows an apparatus wherein a barrel having glass members such as lenses accommodated therein is placed in a housing of a closed structure and wherein the inside of the barrel is filled with an inactive gas.
In ArF excimer lasers having an emission wavelength of about 193 nm, there are plural oxygen (O2) absorption bands in this wavelength region. The inactive gas purging is performed to an optical system to hold the oxygen concentration in the optical path at a very low level and, on the other hand, the purity of the emission wavelength is increased to provide a wavelength of very small absorption, as an exposure beam.
When F2 excimer lasers are used for an exposure process, the following problems arise:
It is known that, in the vacuum ultraviolet region near 157 nm, there is a continuous absorption band to oxygen. The characteristic differs from the region in which an absorption band near 193 nm (ArF excimer laser) is present discontinuously. It is, therefore, impossible to choose an exposure wavelength of very small absorption, as an ArF excimer laser.
Further, it is known that, in the vacuum ultraviolet region near 157 nm, there is a continuous water vapor absorption band, which is absent near 193 nm.
In addition thereto, it is known that vacuum ultraviolet rays near 157 nm are easily absorbed by ammonia (NH3), carbon dioxide (CO2) or an organic gas, for example. Thus, the light absorption along the exposure light path, which may be disregarded in the conventional exposure process using ultraviolet rays not longer than 160 nm, increases very much. This may cause a large decrease of the throughput of the apparatus.
A chemical filter may be used to prevent the supply of such a light absorptive material into the apparatus. However, replacement filters or maintenance thereof requires stopping the operation. This may cause a decrease of the throughput of the apparatus.
Further, when the concentration of such a light absorptive material in the light path changes during the exposure operation, a change (error) occurs in the actual exposure amount relative to a desired exposure amount. This may cause not only a decrease of the throughput but also a large decrease of the exposure amount control precision.
Furthermore, for exposure of a pattern linewidth of 100 to 70 nm, deformation of the apparatus due to a temperature change should be reduced to a low level, to improve the pattern overlay precision. Additionally, reduction of a measurement error due to any fluctuation of various measuring systems accommodated in the apparatus or reduction of changes in characteristic of the optical system due to a temperature change, have to be accomplished as well.
Therefore, the environment control related to the optical path gas components, from the standpoints of efficiency of the optical system, the exposure amount control and the product deposition to the optical system, on one hand, as well as the temperature control of the exposure apparatus from the standpoints of thermal deformation, fluctuation of measuring systems and optical characteristics, on the other hand, should be accomplished simultaneously. In conventional exposure apparatus, both the environment control and the temperature control such as described above are not satisfactorily accomplished.
It is accordingly an object of the present invention to provide an exposure apparatus and/or a device manufacturing method using the same, by which the gas purity in an ambience can be maintained to assure an exposure process of higher throughput.